https://leetcode.com/problems/first-unique-character-in-a-string/public class Solution {    public int firstUniqChar(String s) {        int[] index = new int[26];        for (int i=0; i<s.length(); i++) {            int ch = s.charAt(i) - ‘a‘;            if (index[ch] == 0) {                index[ch] = i + 1;            }            else if (index[ch] > 0) {                index[ch] = -1;            }        }        int ret = s.length() + 1;        for (int i=0; i<26; i++) {            if (index[i] > 0 && index[i] < ret) {                ret = index[i];            }        }        if (ret == s.length() + 1) {            return -1;        }        else {            return ret - 1;        }    }}